Accessories such as water pumps and cooling fans are in common use in vehicles such as automobiles and trucks with internal combustion engines. The accessories are typically driven by a belt directly or indirectly attached to the crankshaft of the engine and thus operate at some percentage of engine speed. The water pumps have an impeller that is used to circulate the engine coolant from the engine to the radiator and back in order to keep the coolant within acceptable temperature limits. Cooling fans have a fan with blades that supplies atmosphere air to a radiator, for example, to cool the engine coolant.
Efforts are being made today to reduce the power consumption of engine accessories, such as water pumps and fan drives, in order to improve fuel economy and reduce emissions. It would thus be preferable if such accessories could be made to operate with less power, or only when needed, in order to reduce the load on the engine and, in turn, improve fuel economy and reduce undesirable emissions from the engine.